The present invention relates to a position detecting apparatus for detecting the position of an optical head of the optical disk apparatus.
In case of detecting the position of the optical head in the optical disk apparatus, position determining is needed to be accurate in milimeter or smaller units. The conventional optical-detecting device employs the principle of grid superposition to achieve the position determination. According to the position determination, an optical scale having a grid pattern is arranged opposite to a mask which also has the same grid pattern as that of the optical scale. The mask is fixed while the optical scale is moved together with the optical head. A light receiving element is attached to the mask, opposite to the optical scale, and serves to convert the light, which enters into it through the optical scale and the mask, to an electric signal. Since the condition, under which the grid pattern of the optical scale is superposed upon that of the mask, is changed according to the movement of the optical scale, the quantity of light entering into the light receiving element changes accordingly, thus causing the light receiving element to provide an electric signal that corresponds to the changing quantity of light. Zero-crossings of the electric signal are detected and an address counter counts up or down in response to the zero-crossings, so that the position of the optical head may be detected according to the contents of the address counter.
The contents of the address counter represent a value corresponding to one pitch of degree of the optical scale, and the electrical dividing method is thus used in order to obtain a measured value smaller than the pitch. According to the method, two signals, different by 90.degree. in phase and obtained from the light receiving element, are divided by dividing resistors to four signals which are shifted by 45.degree. from one another. Each of these signals is converted to a rectangular wave, and pulses are formed corresponding to the rising and falling edges of the rectangular wave. A scale, resulting from dividing one pitch to eight equal parts, is thus obtained according to the method.
The conventional position-detecting device, as described above, needs a more complicated means to realize the electrical dividing method and also requires that the pitch of the optical scale be made substantially smaller because measuring pulses must be provided to correspond to the finely-divided scale.